Photooxidation of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) sensitized by N-methylquinolinium tetrafluoborate (NMQ(+)BF(4)(-)) has been investigated in O-2-saturated acetonitrile solutions. Nearly 100% oxidation of DBT and DMDBT was observed, and the oxidized products are predominantly composed of sulfoxides and sulfones, which are formed via photoinduced electron transfer (ET). Such ET processes were studied with fluorescence quenching of NMQ(+), time-resolved transient absorption measurement, and ESR experiments. The fluorescence of NMQ(+) is efficiently quenched by DBT and DMDBT via diffusion-controlled processes, with bimolecular quenching constants of 1.6 x 10(10) M-1 s(-1) for DBT and 2.3 x 10(10) M-1 s-1 for DMDBT. The electron-transfer nature of the quenching is evidenced by the transient absorption measurement of the neutral radical NMQ(+), which is formed by electron transfer from the substrates (DBT or DMDBT) to the excited singlet state of NMQ(+). The ESR spectra of the superoxide radical anion (O-2(.-)) trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the photooxygenation of DBT and DMDBT as well as their sulfoxides manifest that O-2 traps an electron from NMQ(.) to form O-2(.-). The fact that the formation of sulfoxides and sulfones is greatly suppressed in the presence of benzoquinone (BQ), an efficient electron trap for NMQ(.) and O-2(.-), further indicates an ET process in the photooxidation of DBT and DMDBT. As inferred from the control experiments, the role of singlet oxygen (O-1(2)) in the photooxidation is negligible. The intermediates responsible for the formation of sulfoxides and sulfones have been examined in detail.